1. Field of the Invention
The present invention is directed to a computed tomography apparatus of the type having a gantry with an X-ray source and an X-ray detector thereon.
2. Description of the Prior Art
In practice, it is increasingly necessary in examination of a patient with a computed tomography apparatus to implement X-ray diagnostic examinations, for example in the form of fluoroscopy examinations, digital radiography or digital subtraction angiography, without time delay and without repositioning the patient. To this end, it is advantageous to provide an additional X-ray detector that is irradiated in the course of these types of examinations, which supplies corresponding signals. As little structure as possible, in the already complicated structure of the computed tomography apparatus should be modified for this purpose. In particular, the employment of a second X-ray source should be avoided and the examination in the gantry should ensue without large displacement paths of the patient support. The gantry is the scanner unit of the apparatus that rotates around the patient wherein the X-ray source and the X-ray detector are arranged. One conceivable solution would be to arrange the additional X-ray detector in the beam path preceding the X-ray detector employed, in the course of standard CT examinations. Since, however, the additional X-ray detector would absorb a considerable part of the incident X-rays, this approach would involve the disadvantage of a greatly increased patient dose. Such an arrangement thus is not an acceptable solution. Due, further, to the planar implementation of the X-ray detector, that already can be employed only as a flat detector given arrangement thereof in the gantry, the X-ray detector employed for the CT examinationxe2x80x94which is implemented as a curved arrayxe2x80x94would have to be offset considerably toward the outside in order to be able to provide an additional detector at all, with a given gantry diameter. This involves significant design disadvantages, particularly an increased support ring diameter for the rotating parts, which would lead to significantly larger centrifugal forces.
Known combination systems that, thus, allow standard computed tomography examinations as well as X-ray diagnostics have an additional X-ray tube and a separate detector that are employed together for the x-ray diagnostics. It is also known to employ a patient bed with built-in detector panel from which signals for the x-ray diagnostics signals are supplied.
U.S. Pat. No. 6,198,790 discloses such an X-ray diagnostics apparatus with a computed tomography system that has a first X-ray tube secured to a live ring that emits a fan-shaped payload ray beam and a detector array lying thereopposite. A second X-ray tube is secured to the live ring at a right angle relative to the first X-ray tube, a matrix-shaped X-ray detector being arranged at the live ring lying opposite the second X-ray tube.
U.S. Pat. No. 6,304,627 discloses an X-ray diagnostic installation wherein an exposure unit has a radiation transmitter and a line detector lying opposite one another that are rotatable around an exposure region. A support mechanism with a support plate for the examination subject also is provided. The X-ray diagnostic installation has a further radiation receiver allocated to it that, proceeding from a standby position, can be adjusted into an exposure position for receiving an X-ray beam emanating from the radiation transmitter of the computed tomography apparatus.
An object of the present invention is to provide a computed tomography apparatus that eliminates the initially described disadvantages and allows the implementation of X-ray diagnostic examinations in the gantry of the computed tomography apparatus.
This object is achieved in accordance with the invention in a computed tomography apparatus of the type initially described that has at least one further, curved solid-state radiation detector in the gantry that can be moved out of the beam path of the X-ray source.
The invention employs a displaceably seated solid-state radiation detector in the gantry that is curved, so that it can be introduced as needed into the gap between the inner ring of the gantry and the X-ray detector adjacent thereto, that is likewise curved and serves for the actual CT exposure, so that the displaceably seated solid-state detector is positioned in the beam path of the X-ray source that is already present. Due to the curvature, the extremely small available space can be advantageously utilized. Additionally, nothing about the structure of the gantry needs to be changed, insofar as the actual CT X-ray radiation detector need not be radially offset farther toward the outside due to the curvature of the solid-state radiation detector. A curved solid-state radiation detector can be manufactured using an adequately thin, and thus flexible, detector carrier.
The inventive computer tomography apparatus thus allows switching from the actual CT examination to an x-ray diagnostic examination by means of a simple, as needed introduction of the additional solid-state radiation detector without time delay and without patient repositioning, with the same X-ray tube being employed for both examination modes. All of this is possible without excessive modifications of the actual gantry structure, since the existing, extremely small available space is optimally utilized due to the curved embodiment.
It is especially advantageous, due to the arrangement of the additional X-ray detector for x-ray diagnostics within the gantry, that, by a suitable rotation of the gantry, the examination can ensue in any arbitrary transillumination direction that is most expedient for the desired diagnosis. This enables a manipulation that is just as flexible as a C-arm, which would not be the case for an additional X-ray detector built into the patient bed.
As described, it is expedient when the solid-state radiation detector can be positioned in front of the X-ray detector, i.e. can be introduced into the gap between inner gantry ring and the CT X-ray detector.
Inventively, the bending radius r of the detector should be in the range rxe2x89xa7a and rxe2x89xa7b/2, whereby a is the spacing of the X-ray detector from the X-ray source and b is the inside diameter of the gantry.
In an embodiment of the invention the solid-state radiation detector is movable into the beam path by displacement along a circular path around the rotational axis of the gantry. In this embodiment, thus, the solid-state radiation detector is quasi-tangentially inserted into the beam path along the circular path proceeding from the side. Expediently, the rotational axis around which the circular path displacement motion ensues coincides with the rotational axis of the gantry.
In an embodiment of the invention, an alternative the solid-state radiation detector can be moved into the beam path by axial displacement parallel to the rotational axis of the gantry. Which motion alternative is selected is ultimately dependent on the nature of the space relationships in the gantry and, in particular, on how large the solid-state radiation detector itself is. In any case, suitable displacement and guide means, for example slide or glide rails, on which the solid-state radiation detector is seated and guided are provided for the movement of the solid-state radiation detector, is a suitable drive, for example in the form of a servo motor or motor actuator or the like. It is important that these motion and drive components are as small as possible so that they do not require an unnecessarily large amount of space.
In a further embodiment of the invention, the electronic components serving for the drive and the readout of the pixels of the solid-state radiation detector are not arranged at the front edge of the solid-state radiation detector (xe2x80x9cfrontxe2x80x9d being with reference to the motion direction upon introduction into the beam path). This embodiment is advantageous because the structural height of the curved solid-state radiation detector can be kept low in the region of the front, introduction edge, so that the solid-state radiation detector can be introduced into a gap between the inside gantry ring and the x-ray detector that is narrower than the actual structural height of the curved solid-state radiation detector, including the electronic components arranged at the edge side. These components are expediently arranged at the other three longitudinal edges of the detector at that side facing toward the outside gantry ring.
Due to the curvature of the solid-state radiation detector, it is expedient for the size of the pixels of the solid-state radiation detector to decrease toward the straight edge of the curved solid-state detector. Further, the pixel position, i.e. the position of the rows and columns, can be adapted in tangential and axial directions so thatxe2x80x94with reference to a flat panelxe2x80x94no or only a small amount of distortion and/or pixel anisotropy occur.
Further, a cable-free transmission system for transmitting the signals picked up by the solid-state radiation detector to a control device arranged externally of the gantry, and for the reception of control signals by this control device, can be provided. This transmission system can be an optical transmission system, particularly an IR transmission system, or an electromagnetic transmission system in the form of antennas or the like. Expediently, the transmission devices of the CT X-ray detector that are already present should be used, since these are present anyway, and providing additional transmission and reception means is avoided in this way.
Alternatively to the use of an optical or electromagnetic transmission and reception system, a stationary wiper ring and wiper contacts wiping interacting therewith can be provided for the transmission of signals from the solid-state radiation detector to the control device external from the gantry, and for the transmission of corresponding control signals.
As stated, the CT X-ray detector is in fact relatively long but is also very narrow, for which reason the ray field of the X-ray source is likewise narrow. Since the curved solid-state radiation detector serving for the x-ray diagnostics is essentially rectangular and clearly wider, it is necessary that the ray field can be appropriately varied for the utilization of the entire detector area. To this end, a diaphragm device allocated to the X-ray source is provided, this being controllable via the control device, so that the gating of the ray field can be varied in a simple and fast way dependent on the examination to be implemented. The same is true of the variation of the X-ray dose or of the amplitude of tube voltage and tube current, which are likewise controllable for variation via the control device dependent on the examination to be implemented, and thus on the detector to be irradiated.